1. Field of the Invention
The present invention relates to a pulse radar device which measures a distance to a target object.
2. Description of the Related Art
Up to now, there has been well known a pulse radar device which transmits a transmit signal from an antenna and receives a reflection signal that reflects a plurality of target objects (target) to detect the presence of the respective target objects and to measure the positions of the target objects.
In the pulse radar device, signal processing is conducted by using the received reflection signal, but in fact, since the reflection signal is received from a radome located in the pulse radar device, there is required a signal processing unit for separating and extracting the target object.
FIG. 26 is a block diagram showing a conventional pulse radar device disclosed in, for example, Japanese Patent Application Laid-open No. Hei 7-72237. In FIG. 26, the pulse radar device includes a transmitting means 2601, a control means 2602, a receiving means 2603, sampling means 2604, adding/storing means 2605 and judging means 2606.
The transmitting means 2601 periodically outputs (transmits) a pulse-like signal (transmit signal) toward the target object in accordance with an output command from the control means 2602.
The transmit signal transmitted from the pulse radar device is reflected by a plurality of target objects and then inputted to the receiving means 2603. The receiving means 2603 continuously receives a receive signal (pulse signal) including the reflection signals from the respective target objects and binary-codes the receive signal.
The sampling means 2604 samples the binary-coded pulse signal to 0 or 1 in accordance with the command of the control means 2602, and inputs the sampling value at each sampling point to the adding/storing means 2605 corresponding to the respective sampling points.
The adding/storing means 2605 adds a sampling value in accordance with the number of times of transmission of a transmission signal from the transmitting means 2601.
The judging means 2606 compares the respective normalized addition values obtained by dividing the respective addition values by the adding/storing means 2605 by the number of times of addition with a predetermined detection threshold value (threshold value) to judge the presence/absence of a reflection signal from a target object on the basis of the magnitude relationship of a comparison result. Also, the judging means 2606 judges the presence/absence of the target object in accordance with the judgment result of whether the reflection signal is present or absent, and calculates the distance to the target object in accordance with the judgment result.
In the conventional device, in general, a transmit pulse width is set to 66.7 ns (corresponding to 10 m in distance).
FIG. 27 is a timing chart showing transmit/receive waveforms together with a reflection wave and a leakage waveform.
For example, in the case where the target object exists within 10 m, and the isolation of transmit/receive is low such that a leakage wave exists or a reflection signal from a radome exists in the receive signal, the receive signal is detected as shown in FIG. 27, in a state where the leakage signal, the reflection signal (not shown) from the object fixed onto the pulse radar device such as a radome, a noise signal such as a noise (not shown) which exists even in non-transmission, and the reflection signal from the target object are superimposed on each other in waveform.
Therefore, even if the detection threshold value is set on the basis of only the receive level (noise level) in the non-transmission, and the reflection signal from the target object is going to be discriminated, only the rising of the leakage waveform or the reflection waveform such as the radome can be detected, and therefore the rising of the reflection waveform from the target object cannot be detected.
From the above-mentioned viewpoints, in the conventional device shown in FIG. 26, it is difficult to detect the target object at a short distance and measure the distance.
The countermeasures for solving the above problems are disclosed in various documents up to now.
For example, there is discloses a method with a transmit pulse width set to 350 ps (far shorter than that in the above-mentioned conventional device) in a document of 1998 (W. Weidmann and D. Steinbuch, xe2x80x9cHigh Resolution Radar for Short Range Automotive Applicationsxe2x80x9d, 28th European Microwave Conference Amsterdam, 1998).
However, this method cannot be used within the existing radio wave law because the leakage waveform and the reflection waveform of the target object are superimposed on each other only when the distance to the target object is about 5 cm or shorter, and the above-mentioned problem is almost solved but a wide frequency occupied band width is required.
Also, Japanese Patent Application Laid-open No. Hei 10-62518 discloses a method of canceling the leakage waveform by using the transmit waveform.
However, this method is difficult to cope with a difference in the period of time from the transmission of the transmit signal to the receive of the leakage waveform due to a difference of the use conditions or with a difference in the magnitude of the leakage waveform, and must adjust the pulse radar device in accordance with the circumstances.
As described above, in the conventional pulse radar device, in the case where a noise signal exists in the receive signal, since the receive signal is detected in a state where the noise signal and the reflection signal from the target object are superimposed on each other in waveform, even if the detection threshold value is set only on the basis of the receive level (noise level) in the non-transmission to discriminate the reflection signal from the target object, the rising of the reflection waveform from the target object cannot be detected, resulting in such a problem that it is difficult to measure the distance.
Also, when a method of shortening the transmit pulse width to 350 ps is applied, since the leakage waveform and the reflection waveform of the target object are superimposed on each other only when the distance to the target object is about 5 cm or shorter, the above-mentioned problem is almost solved, but there arises such a problem that the wide frequency occupied band width is required, and this method cannot be: used within the limits of the existing radio wave law.
In addition, when a method of canceling the leakage waveform by using the transmit waveform is applied, it is difficult to cope with a difference in the period of time from the transmit of the transmit signal to the receive of the leakage waveform due to a difference in the use conditions, and with a difference in the magnitude of the leakage waveform, and there arises such a problem that the pulse radar device must be adjusted in accordance with the circumstances.
The present invention has been made to solve the above-mentioned problem, and therefore an object of the present invention is to provide a pulse radar device which is capable of discriminating a reflection waveform from a target object to measure a distance to the target object even if a noise signal exists in a receive signal.
Another object of the present invention is to provide a pulse radar device which is capable of accurately detecting a target object within the limits of the existing radio wave law, in accordance with a change in a receive signal due to a phase difference of a leakage signal between the transmit and the receive, a reflection signal from the object fixed to the pulse radar device such as a radome, a noise signal such as a noise component which exists even in the non-transmission with respect to the reflection signal from the target object.
Still another object of the present invention is to provide a pulse radar device which is capable of coping with a difference in the period of time from the transmit of a transmit signal to the receive of a leakage waveform due to a difference in the use conditions and with a difference in the magnitude of the leakage waveform without adjusting the pulse radar device.
According to the present invention, there is provided a pulse radar device, comprising a timing control unit that controls a transmit interval of a pulse signal, a transmitting unit that transmits the pulse signal, a receiving unit that receives a receive signal including a reflection signal component from a target object and a noise component. And further provided is a receive signal change detecting unit that detects a change in the assembly of the receive signals of the assembly of the pulse signals as many as a given number of times of transmissions which are transmitted during a first transmit interval and the assembly of the pulse signals as many as a given number of times of transmissions which are transmitted during a second transmit interval. And further provided is a reflection signal rising detecting unit that detects a rising time point of the reflection signal component on the basis of the change in the assembly of the receive signals, and a ranging/detecting unit that obtains a distance value on the basis of the rising time point of the reflection signal and judges the presence of the target object.
Therefore, even if a leakage signal component or a reflection signal from a radome or the like exists in the receive signal, the target object can be accurately detected.